Folders Shared by Default in OS X Mountain Lion

When you add users in the Users & Groups System Preferences pane in OS X Mountain Lion, OS X automatically does two things behind the scenes to facilitate file sharing: It creates a set of folders, and it makes some of them available for sharing.

Each time you add a Managed, Standard, or Administrator user, OS X creates a Home folder hierarchy for that user on the Mac. The user can create more folders (if necessary) and also add, remove, or move anything inside these folders. Even if you create a user account solely to allow him or her to exchange files with you, your Mac automatically creates a Home folder for that user.

Unless you, as the owner of your Mac, give permission, the user can’t see inside or use folders outside the Home folder (which has the user’s name), with only three exceptions: the Shared folder in the Users folder, the top level of other user account folders, and the Public folders in every other user’s folder, as well as the Shared folder within the Users folder.

A description of the latter follows:

Public: A Public folder is located inside each user’s folder. That folder is set up to be accessible (shared) by any user who can log in to the Mac.

Furthermore, any user can log in (as a guest) and copy things out of this folder as long as she knows your Mac’s IP address, even if she doesn’t have an account on this Mac at all. Files put into the Public folder can be opened or copied freely.

It’s not hard for someone to obtain your IP address. For example, when you visit most web pages, your IP address is saved to that site’s log file. So be careful what you put in your Public folder.

This is also an excellent reason to employ a firewall. Mountain Lion has an excellent software implementation available via the Firewall tab in Security & Privacy System Preferences, and most routers (for example, AirPort Extreme) include a hardware firewall.

Inside each user’s Public folder is a Drop Box folder. As the name implies, this folder is where others can drop a file or folder for you. Only the owner can open the Drop Box to see what’s inside — or to move or copy the files that are in it. Imagine a street-corner mailbox; after you drop your letter in, it’s gone, and you can’t get it back out.

Shared: In addition to a Public folder for each user, OS X creates one Shared folder on every Mac for all users of this Mac. The Shared folder isn’t available to guests, but it’s available to all users who have an account on this machine.

You find the Shared folder within the Users folder (the same folder where you find folders for each user). The Shared folder is the right place to put stuff that everyone with an account on this Mac might want to use.